If you use GMail as Web application you access your GMail account from the browser so it is covered by the Web Shield and the Network Shield, not the Mail Shield.
The other option is to access your Gmail as POP3 using a mail client (es. Thunderbird). In that case you have to disable the SSL encryption in the mail client server settings because SSL is provided by Avast Mail Shield. Then you can go in the "expert settings" of the Mail Shield and "SSL accounts", there you can see the list of mail accounts/servers that Mail Shield is monitoring and one should be GMail.